This invention relates to a high voltage cable termination, and more particularly, to a connection interface for connecting power transmission cable to other apparatus such as an overhead electric power transmission line, gas-insulated substation, or oil-insulated power transformer.
A cable used for transmitting electric power must be connected to other elements of the electric power system to be useful. This usually involves transitions between the cable and air-insulated, SF.sub.6 -insulated, or other fluid dielectric-insulated components. In an underground electric power cable, the high voltage conductor and the concentric grounded shield are separated at most by a few centimeters of dielectric, usually a dielectric fluid impregnated paper or paper-polypropylene (PPP) laminate (both cases are known as laminar dielectric cable), or by an extruded solid dielectric such as cross-linked polyethylene (XLPE), thermoplastic polyethylene (PE), or a filled ethylene propylene rubber (EPR).
The very large electric field in an electric power cable is radial, between the high voltage conductor and the concentric ground shield. The electric fields at which power cables operate are among the highest of any power system components. To connect such a power cable to external apparatus, such as air-insulated, SF.sub.6 -insulated, or bulk oil-insulated apparatus (all of which support lower electrical stresses than those caused by the electric field of a typical power cable), the ground of the cable must be spaced from the cable conductor sufficiently far and in such a manner that where the cable is connected to the apparatus, the dielectric fluid which insulates the apparatus can withstand the electrical stresses imposed by the cable termination between its high voltage terminal and ground. This requires increasing the spacing between the conductor and the ground from a few centimeters in the cable to the range of meters in the case of a transition to air-insulated apparatus. For example, if a high voltage cable were to be terminated by simply cutting it perpendicular to its length, the high voltage conductor would be separated from the ground shield by only a few centimeters across the surface of the cut dielectric. At the electrical stresses employed in high voltage power cable, the air, oil, SF.sub.6, or other fluid dielectric along this surface would suffer dielectric breakdown at a fraction of the voltage on the conductor. If the coaxial ground shield were terminated with a circumferential cut and the dielectric and conductor were extended some distance beyond, the termination may fail. The electric field at the edge of the shield termination would be so high as to cause short-term failure of the cable dielectric. In addition, unless the conductor extended a very large distance beyond the shield termination, the breakdown of the dielectric fluid adjacent to the shield termination would lead to breakdown between the shield and the conductor along the surface of the cable dielectric.
Laminar dielectrics, in particular, have very high dielectric strength (i.e., they support very large electric fields) in the radial direction (i.e., perpendicular to their surface) but have comparatively low dielectric strength in the longitudinal direction (axially, or along their surface). A longitudinal component of the electric field is an inevitable result of any attempt to increase the separation between the conductor and ground. Thus for laminar dielectric cables, the means for achieving the necessary separation between the conductor and ground shield requires careful design to assure that the longitudinal component of the electric field does not exceed a safe value.
Electric power cables are used to transfer large amounts of electrical power by conducting large electric currents at high voltages. Cable conductors, such as copper and aluminum, have electrical resistance, and cable dielectrics have dielectric losses, all resulting in substantial amounts of heat being generated per unit length of cable. The laminar or extruded dielectric of a power cable has limited ability to operate at high temperatures. The ultimate limit on the power transfer of a power cable system is normally the maximum allowable temperature of the cable dielectric, above which the operating life of the cable degrades rapidly. A power cable system is designed so that at its rated power, the heat generated by the cable can be dissipated safely into the soil in which the cable is buried without exceeding the maximum allowable temperature for the cable dielectric. However, the cable termination represents a thermal environment which differs substantially from that in which the majority of the cable operates. The elements of a conventional cable termination act as a substantial thermal resistance between the cable conductor within the termination and the exterior environment in which the heat must be dissipated.
The prior art has long been aware of the need for care in the design of high voltage cable terminations (or potheads). Conventional transmission-class power cable pothead designs usually employ two separate mechanisms for the purposes of (1) expanding the distance between the cable conductor and ground while limiting the longitudinal component of the electric field and (2) creating an acceptably uniform electric field along the exterior surface of the termination between the high voltage conductor and ground.
Conventional cable termination designs for laminar dielectric cables expand the cable shield diameter over a built-up section of cable dielectric (a "stress cone" in language of those of ordinary skill in the art) in order to decrease gradually the electric field in the cable dielectric and create a manageable and substantially constant longitudinal component of the electric field in the region of the stress cone. For laminar dielectric cables, the stress cone typically is implemented through the use of an oil-impregnated paper roll with a "log-log" taper at one end. This roll is tightened over the cable dielectric, and the concentric ground shield is arranged to expand over the log-log taper to increase the separation between the conductor and the shield while creating a safe and nearly constant longitudinal component of the electric field. This mechanism achieves an acceptable electric field configuration at the termination of the ground shield. However, manufacture of the log-log taper on a very large paper roll is difficult and risky. Moreover, installation of the very large paper roll without distorting the log-log taper is very difficult, and even slight distortions can cause failure of the termination. In addition to the stress cone, another device in the form of a capacitor stack connected between the high voltage terminal and ground and placed along the inner surface of the cable termination enclosure (usually porcelain), is often used to achieve acceptable electric field grading at the external surface of the termination. Since, in principle, a capacitor stack provides nearly uniform grading of the electric field between its ends, a capacitor stack might be connected from the termination of the ground shield to the cable conductor at the top electrode of the termination to achieve the dual purposes of uniform field grading at the external pothead surface and establishing a very well controlled longitudinal electric field within the cable dielectric. However, conventional wound capacitor technology results in an electric field which changes rapidly over short distances along the stack rather than continuously along its length. In conventional cable terminations, such a step-wise grading is sufficiently "smoothed" by the high dielectric constant and thickness of the porcelain enclosure to provide acceptable grading in the fluid dielectric (e.g., air) along the exterior surface of the cable termination; however, such substantially step-wise grading will result in unacceptable longitudinal stresses along the cable surface of the stress cone and within the dielectric of a laminar cable dielectric.
One solution to the dual problems of controlling longitudinal electric field in the dielectric and controlling the electric field in the air along the exterior surface of a cable termination is described in U.S. Pat. No. 4,179,582 to Garcia. This patent suggests the use of the conventional stacked capacitor grading to control the electric field along the exterior surface of the termination. Moreover, rather than use a paper roll with a log-log taper, Garcia suggests the use of a "short-stack" of capacitors constructed so as to provide uniform (as opposed to stepwise) longitudinal grading, although the nature of the capacitors appropriate to the task is not discussed. This design is relatively expensive to manufacture and assemble and, moreover, is highly labor intensive because of the need to build the termination on site.